Recently, as a light source used in the field of optical information processing, research and development of a producing visible laser, such as green and blue laser has been proceeding. As one kind of visible laser, a wavelength converting laser device which converts near-infrared laser into short-wavelength laser by wavelength conversion is known.
Generally speaking, a wavelength converting laser device includes, a wavelength converter including a nonlinear optical material disposed either inside or outside of an optical resonator for a laser diode or a solid-state laser. A laser (a fundamental wave) generated by the optical resonator is output as a second harmonic wave which has a one-half the wavelength (double the frequency) of the fundamental wave as a result of propagation of the fundamental wave through the nonlinear optical material.
Published Japanese patent application S63-121829 provides an example of this sort of wavelength converting laser device. Especially in a wavelength converting laser device of embodiment three and FIG. 3 of that publication, an optical resonator is provided by connecting a laser diode and a wavelength converter and forming a reflector at the one facet thereof in an integrated structure. High conversion efficiency is achieved through increased intensity of light by placing the wavelength converter within the optical resonator, even if the pump source is small and low power and produces continuous-wave light, as laser diodes.
However, there is a limit to the power that can be obtained with such a wavelength converting laser device, since this wavelength converting laser device includes a laser diode and a wavelength converter. It is impossible to produce a Watt-level laser (fundamental wave) with a narrow stripe semiconductor lasers having a small light emitting area. On the other hand, since a mode control of a horizontal transverse mode horizontal to an active layer (horizontal transverse mode) is not achieved in a broad-area laser diode having a large light emitting area and Watt-level power, that laser oscillates with multitude transverse modes and has low beam quality. Where this sort of broad-area laser diode is used, it is impossible to obtain a highly efficient wavelength converting laser device since a laser (fundamental wave) with low beam quality has insufficient coupling efficiency or insufficient angle-turned phase matching with the wavelength converter.
As described above, it is impossible for previous wavelength converting laser device to produce a laser having Watt-level power and high efficiency at the same time.
It is an objective of this invention to solve the above mentioned problem of the previous wavelength converting laser device.